The present invention relates to an aqueous latex coating composition for application to polymer film by which the film is rendered heat sealable.
Uncoated polymer films such as oriented polypropylene have very high heat-sealing temperatures and an extremely narrow heat sealing range. Especially in the case of uncoated, oriented polypropylene, such films exhibit a tendency to disorient and shrink when the requisite sealing temperatures are applied to the surface thereof. Attempts have been made to produce coatings for thermoplastic film substrates employing various hydrocarbon resins and mixtures of resins and the like to provide, for example, improved heat seal properties, but in many instances such polymeric coatings have had deleterious effects on the final coated film product such as poor blocking characteristics, inferior optical properties, inadequate heat stability and poor aging characteristics.
The use of latices of thermoplastic polymers with or without minor amounts of other materials, either soluble or insoluble in water, to form a heat sealable coating for thermoplastic film is known. U.S. Pat. No. 3,297,477 describes an aqueous latex heat sealable coating composition for application to a monoaxially or biaxially oriented thermoplastic film such as polypropylene. The coating composition contains from about 5 to about 50% by weight of a homogeneous composition of (1) from 50 to 80%, based on the weight of the composition, of an interpolymer of from 50 to 60 wt percent of a C.sub.1 -C.sub.4 alkyl methacrylate, from 50 to 40 wt percent of C.sub.1 -C.sub.4 alkyl acrylate, and from 1 to 5 wt percent of an acid selected from the group consisting of methacrylic, acrylic and itaconic acid, the individual monomer units not exceeding a total of 100%; (2) from 50 to 20%, based on the weight of the composition, of a compound selected from the group consisting of an adduct of rosin and an alpha-beta unsaturated dicarboxylic acid, and the partial esters of said adduct with a polyhydric alcohol; and (3) from 1 to 5%, based on the weight of the composition, of a wax selected from the group consisting of paraffin wax and microcrystalline hydrocarbon wax, the components of the composition not exceeding a total of 100% by weight.
Another heat sealable coating composition of the latex type, also useful for application to oriented thermoplastic films including those based on polypropylene, is described in U.S. Pat. No. 4,291,090. The coating composition comprises an aqueous dispersion containing, in substantially particulate form, a copolymer formed from (a) at least one alpha, beta-unsaturated carboxylic acid in an amount of from 0.1 to 10% by weight of the copolymer; (b) at least one acrylic ester selected from the group consisting of alkyl acrylates and alkyl methacrylates whose alkyl groups contain from one to twelve carbon atoms; and, (c) at least one copolymerizable hydroxyalkyl ester of alpha, beta-unsaturated carboxylic acids, the amount of component (c) being from 0.1 to 80 percent by weight of the copolymer.
The foregoing coating compositions and those of similar type suffer from deficiencies in several properties including jaw release and hot slip. These two closely related properties are related to the thermoplastic nature of the coating. When the surface of the film is heated sufficiently to achieve heat sealability, the non-sealed surface of the film tends to adhere to the heated sealing jaws and/or to stick to non-heated metal surfaces of the packaging machinery thus, preventing the use of commercially desirable high operating speeds ("machinability"). It has been observed that the addition of substantial amounts of fine particle size colloidal silica to these and other known acrylic copolymer latices as a slip agent to improve their machinability causes the coatings to be extremely hazy and impairs their heat sealability.
U.S. Pat. Nos. 3,753,769 and 4,058,645 each discloses heat sealable coating compositions for oriented polymer films, e.g., those derived from polypropylene, which overcome the machinability deficiencies referred to. The composition of U.S. Pat. No. 3,753,769 contains as a film forming component a resin consisting essentially of an interpolymer of (a) from about 2 to about 15 parts, and preferably from about 2.5 to about 6 parts by weight, of an alpha, beta monoethylenically unsaturated carboxylic acid selected from the group consisting of acrylic acid, methacrylic acid and mixtures thereof, and (b) from about 85 to about 98, and preferably from about 94 to about 97.5 parts by weight, of neutral monomer esters, said neutral monomer esters preferably comprising (1) methyl acrylate or ethyl acrylate and (2) methyl methacrylate. The coating formulation of U.S. Pat. No. 4,058,645 comprises a mixture of a resinous interpolymer consisting essentially of (a) from about 2 to about 15 parts, and preferably from about 2.5 to about 6 parts by weight, of an alpha, beta monoethylenically unsaturated carboxylic acid selected from the group consisting of acrylic acid, methacrylic acid and mixtures thereof, and (b) from about 85 to about 98, and preferably from about 94 to about 97.5 parts by weight, of neutral monomer esters preferably comprising methyl acrylate or ethyl acrylate and methyl methacrylate, the interpolymer being in admixture with from about 2 to about 15 parts by weight of an aqueous-alkali soluble rosin derivative. The coating compositions of both patents are intended to be applied to a substrate polymer film as ammoniacal aqueous solutions containing substantial amounts, e.g., from 30-60 weight percent, of a small particle size colloidal silica as a hot slip agent. Despite such high particle loadings, the heat sealability and high optical clarity of these coatings are not significantly impaired.